1. Field of the Invention
The present invention relates to an optical system for preserving a state of polarization of light beam and, more particularly, to an optical system for stably detecting a light beam reflected or transmitted by an optical memory.
2. Description of the Related Art
A data recording/reproducing apparatus for optically reproducing recorded data and for optically recording data has been developed, which uses an optical disk, an optical card, or an optical tape as an optical memory for storing data. An optical card, for example, generally comprises a recording layer formed on a transparent substrate and a transparent protective layer formed on the recording layer. When data thereon is to be reproduced, for example, a light beam is radiated onto a recording surface by an optical head through this substrate or the protective layer. Then, the light beam is intensity-modulated in accordance with the data on the recording surface, and is reflected thereby. The reflected light beam is picked up by an optical head arranged in the data recording/reproducing apparatus and is converted into an electrical signal to be used as a readout signal or a servo control signal. It should be understood that "optical head" is, in the case of this description, a general term used to denote a set of devices for radiating a light beam on an optical memory, detecting the light beam from the optical memory by means of an optical detector, and obtaining a readout signal for reproducing data and a servo control signal used for focusing, tracking, and the like.
A conventional optical head includes, for example, a polarization beam splitter so as to split a light beam radiated onto an optical memory from a light beam reflected by the optical memory, and a .lambda./4 plate. In the case of the conventional optical head, a light beam emitted from a light source is directed while being linearly polarized (p-polarized light) so as for the beam to be transmitted through the polarization beam splitter at a transmittance of 100%. The light beam transmitted through the polarization beam splitter is converted into circularly polarized light by the .lambda./4 plate arranged on its optical path, and is radiated onto the recording surface of the optical memory. The light beam reflected by the recording surface is transmitted through the .lambda./4 plate again, is converted into linearly polarized light beam (s-polarized light) having its polarization direction rotated by 90.degree. with respect to incidence beam polarization direction. Then, the s-polarized light beam is returned to the polarization beam splitter. The polarization beam splitter reflects the s-polarized light beam at a reflectivity of 100%. The s-polarized light beam radiates onto the optical detector, where it is converted into an electrical signal in accordance with the intensity of the light beam. The electrical signal is used as a readout signal and a servo control signal. As described above, a light beam which is reflected by the optical memory and is returned to the polarization beam splitter through the .lambda./4 plate is ideally linearly polarized light (s-polarized light). An optical head comprising such a polarization beam splitter and .lambda./4 plate is disclosed in U.S. Pat. No. 4,451,913.
However, as circularly polarized light beam is radiated onto an optical memory, a light beam reflected by optical memory is elliptically polarized, not circular, because of the effect of the substrate and the protective layer birefringence. As a result, a different elliptically polarized light beam is returned to the polarization beam splitter after transmitting the .lambda./4 plate, and hence the intensity of the light beam reflected by the polarization beam splitter is decreased. A resin material such as polycarbonate is generally used for the substrate and the protective layer of an optical card. Since these resin materials have asymmetrical crystal structures, they are birefringent (i.e. the propagation speeds of polarized light beams having different polarization direction differ from each other). For this reason, when a light beam passes through the protective layer, retardation necessarily occurs. That is, since the phase of one polarized component of the light beam is delayed with respect to the phase of the other polarized component thereof, an elliptically polarized light beam is reflected by the optical memory. If, for example, retardation caused by the substrate or the protective layer birefringence is .gamma., intensity of detected light varies in proportion to cos.sup.2 (.gamma./2). According to cos.sup.2 (.gamma./2), therefore, as retardation .gamma. due to birefringence is increased, the intensity of the detected light is reduced. When .gamma.=180.degree., the light intensity becomes zero. Accordingly, if a retaration due to birefringence is large, a readout signal and servo control signals cannot be stably obtained.
As described above, according to the conventional optical head, a light intensity detected by the optical detector is decreased because of the influences of the substrate and the protective layer birefringence, thereby posing a problem that readout signals and servo control become unstable. Another problem is that the polarization state of a light beam emerging from a recording surface cannot be maintained.